The present invention relates to fuel gas saturators for providing a stable and consistent supply of moisturized fuel gas to a gas turbine during all steady state and transient operating conditions and, in particular, to an instrumentation and control system for adjusting the flow rate of recycle water to the fuel gas saturator to hold constant the ratio of saturator inlet water flow to saturator inlet dry gas flow. In this manner, consistent moisturized fuel properties, particularly the Wobbe number, are maintained within a very narrow range to satisfy gas turbine combustion system requirements.
Generally, a combined cycle fuel gas power plant includes a gas turbine, a steam turbine, a heat recovery steam generator, a fuel superheater and a fuel gas saturator. Dry fuel gas enters the system in the fuel gas saturator, where the fuel gas is saturated with water before entering the fuel gas superheater. After being superheated, the moist fuel gas enters the gas turbine system for combustion. The effluents from the combustion reaction expand in the gas turbine driving a rotor coupled to a generator for generating electricity. The exhaust from the gas turbine enters a heat recovery steam generator, which utilizes the heat from the gas turbine exhaust to generate steam for use in the steam turbine, heat water for use in the fuel gas saturator and to superheat the fuel gas in the fuel gas superheater. The steam generated in the heat recovery system expands in the steam turbine, generating power.
Natural gas fired combined cycles with Dry Low NOx (DLN) combustion systems impose strict requirements on the fuel gas saturation process due to tight fuel specification tolerances, e.g., variables such as heating value, temperature, fuel composition and so forth. If fuel supply conditions deviate excessively from the designed fuel specification, performance will degrade, e.g., dynamic pressure instabilities and high emissions will occur. Ultimately, conditions may degrade sufficiently to cause the system to trip.
Fuel gas saturation has been employed in a number of integrated gasification combined cycle (IGCC) installations over the last two decades. IGCCs are typically designed with a backup fuel, e.g., distillate, to increase plant availability. Since distillate is high in hydrogen content, the distillate combustion system is designed for diffusion operation, which has much higher tolerance to fuel supply Wobbe number variation than the DLN combustion system employed on most modern natural gas fired turbines. The Wobbe number is important for fuel combustion stability and is calculated according to EQUATION 1:       Wobbe    ⁢          xe2x80x83        ⁢    Number    =            Fuel      ⁢              xe2x80x83            ⁢      Lower      ⁢              xe2x80x83            ⁢      Heating      ⁢              xe2x80x83            ⁢      Value      ⁢              xe2x80x83            ⁢              (                  Btu          /          scf                )                            (                  Fuel          ⁢                      xe2x80x83                    ⁢          Temperature          ⁢                      xe2x80x83                    ⁢                      (                          Deg              .                              xe2x80x83                            ⁢              Rankine                        )                    xc3x97          Fuel          ⁢                      xe2x80x83                    ⁢                                    Mol              .                              xe2x80x83                            ⁢              Wt              .                        /            28.96                          )            
The Wobbe number of the fuel gas supplied to the gas turbine tends to vary significantly in IGCC plants, because the fuel composition from the gasification system varies with load and feedstock to the gasifier. The heat source for distillate saturation is the distillate cool down system, which operates at essentially fixed pressure (and hence hot water supply temperature) to the fuel gas saturator across the saturator operating range. Accordingly, the water flow supply to the fuel saturator is constant across the load range, and fuel supply Wobbe number control is not an overriding constraint on the gas turbine combustion system operability or design as it is for DLN premixed combustion systems.
The present invention provides a control method for maintaining fuel moisture level within a narrow range in order to satisfy gas turbine combustion system requirements. Generally, this is achieved by maintaining substantially constant the water-to-dry fuel ratio in the fuel gas saturation column based on measured dry fuel flow and water supply to the saturation column, i.e., recycle water and make-up water, across the fuel gas moisturization system operating range. Also, additional information may be used to adjust the water supply to the saturator column, such as one of the following: gas turbine fuel supply temperature, gas turbine fuel supply moisture content, gas turbine supply fuel composition, and gas turbine fuel supply heating value, to maintain substantially constant the fuel Wobbe number to the gas turbine.
In a preferred embodiment of the invention, dry fuel gas flow to the saturator column is measured and the saturator recycle water flow is summed with the saturator makeup water flow. The recycle water flow is then modulated to hold the water-to-dry fuel ratio substantially constant in the saturator. The substantially fixed ratio thus reduces the variation in Wobbe number, and controls the properties of the fuel entering the gas turbine system. In this preferred embodiment of the invention, the control system may be used in a multi-pressure steam bottoming cycle in which the heat source for fuel moisturization is the gas turbine exhaust gas downstream of the LP (low pressure) evaporator, which for the purposes of fuel moisture and Wobbe number stabilization is operated with a fixed steam pressure.
In another preferred embodiment of the invention, an additional measured or calculated value generating a signal may be used to achieve the desired substantially water-to-dry fuel ratio by providing closed loop feedback to achieve the targeted Wobbe number. The following is a non-exhaustive list of additional measured or calculated signals that may be employed in this manner: gas turbine fuel supply temperature, gas turbine fuel supply moisture content, gas turbine fuel supply composition, and gas turbine fuel supply heating value. Closed loop feedback or open loop water-to-fuel ratio bias based on downstream fuel measurements minimizes gas turbine fuel supply Wobbe number variation during operation of the fuel moisturization system. This embodiment of the invention is preferably for use in applications with a less stable heat source, such as a single pressure steam bottoming cycle operated in sliding pressure mode, a multi-pressure steam bottoming cycle with the LP steam pressure operated in a variable pressure mode, or any other cycle where the saturation water heat source has significant temperature variation.
In a preferred embodiment according to the present invention, there is provided a control system for a gas turbine having a saturator, a dry fuel gas input to the saturator, a water input to the saturator for moisturizing the dry fuel gas in the saturator, and an outlet for providing moisturized fuel gas to the gas turbine, a method for controlling fuel gas saturation comprising the step of maintaining a substantially constant ratio of water input to the saturator to dry fuel gas input to the saturator during premix combustion mode operation of the gas turbine.
In a further preferred embodiment according to the present invention, there is provided an apparatus for supplying moisturized fuel gas to a gas turbine comprising a saturator, a first conduit for supplying dry fuel gas to the saturator, a second conduit for supplying moisturized fuel gas from the saturator to the gas turbine, a third conduit for supplying water to the saturator, and a water flow controller for controlling the flow of water received by the saturator through the third conduit to maintain a substantially constant ratio of water input to dry fuel gas input to the saturator.